Usually, a protective layer (hard coat layer) is given to the surface of various articles for which scratch resistance and abrasion resistance are required, for example, optical recording media such as a CD (Compact Disk) and a DVD (Digital Versatile Disk), magneto-optical recording media, an optical lens, an optical filter, an anti-reflection film, and various display elements such as a liquid crystal display, a CRT display, a plasma display and an EL display.
In many cases, stains such as a finger print, sebum, sweat and cosmetics are adhered to the surface of these articles by user's use of the articles. Once such stains are adheres thereto, they are not easily removed. This is a serious problem, in particular, for optical recording media or optical lenses used to record or reproduce the media since the recording and reproducing of information signals are remarkably obstructed by the adhered stains.
In magneto-optical recording media, a magnetic head runs on an organic protective layer formed on their recording layer. Accordingly, it is required that the abrasion resistance of the protective layer is made high and, simultaneously, the frictional coefficient thereof is made low.
As the method for solving the former problem, suggested are various methods of forming, on the surface of an optical lens or the like, a layer having a nature that stains do not adhere easily to the layer and even if stains adhere to the layer, the stains are easily wiped off, that is, a layer having anti-staining property. Specifically, the following method is adopted in many cases: a method of forming a layer made of a fluorine-based compound or a silicone-based compound on the surface to give water repellency and oil repellency to the surface, thereby improving the anti-staining property.
About the method for overcoming the latter problem, that is, the method for decreasing the frictional coefficient of the surface of a protective layer (hard coat layer), many measures have been suggested so far. Specifically, the following method is used in many cases: a method of forming, on the surface of the protective layer, a film made of a liquid lubricant such as a fluorine-based polymer (for example, perfluoropolyether) or a silicone-based polymer (for example, polydimethylsiloxane), thereby improving lubricity.
Originally, the former anti-staining property and the latter lubricity are entirely different properties. However, it is common to the two that a fluorine-based compound or a silicone-based compound is used as means for giving each of these properties in many cases. Accordingly, problems common to the two are frequently caused when a fluorine-based compound or a silicone-based compound is used to give anti-staining property or lubricity to the surface of a hard coat.
Many fluorine-based compounds or silicone-based compounds are soft. Thus, when these compounds are used, it is very difficult to obtain a sufficient abrasion resistance. In order to overcome such a problem, the following method can be considered: a method of adding an inorganic filler made of SiO2 fine particles or the like to a fluorine-based polymer or silicone-based polymer matrix to make the abrasion resistance high. According to such a method, however, a little improvement is made but a satisfactory abrasion resistance cannot be obtained as far as the fluorine-based polymer or silicone-based polymer is used as the matrix, wherein the inorganic filler is dispersed.
Therefore, the following method is considered: a method of making a protective layer into a lamination structure composed of two or more different layers, making the lower layer into a layer made of a highly hard material, and forming an upper layer made of a fluorine-based compound or silicone-based compound on the surface thereof, thereby giving anti-staining property or lubricity. In this case, it is preferable to make the upper layer, which is made of the fluorine-based compound or silicone-based compound, as thin as possible so as to reflect the hardness of the lower layer in the upper layer, which constitutes the topmost surface of the lamination protective layer. However, in this method, it is very difficult to obtain close adhesion between the lower layer and the upper layer which is made of the fluorine-based compound or silicone-based compound.
As the method for solving the above-mentioned problem about the adhesion, for example, the following method is known: a method of forming a lower layer made of an inorganic material such as SiO2 by such a process as sputtering or sol-gel process; forming, on the surface of the lower layer, an upper layer made of alkoxysilane having fluoroalkyl group by such a process as vapor deposition or solution application; subjecting the resultant to heat treatment in the presence of a very small amount of water content so as to cause dehydration condensation between silanol groups generated by hydrolysis of the alkoxysilane and/or between the silanol groups and hydroxyl groups present in the surface of the lower layer made of SiO2 or the like, whereby the upper layer is fixed onto the lower layer surface through chemical bonds and/or hydrogen bonds.
In this method, it is desired that the lower layer surface has active groups such as hydroxyl groups at a high density. Therefore, the material that can be used in the lower layer is limited to an inorganic compound, in particular, a metal oxide or a metal chalcogenide such as SiO2, Al2O3, TiO2 or ZnS. Even when the lower layer is made of a metal oxide such as SiO2, in order to make adhesion between this metal oxide and the alkoxysilane of the upper layer sufficient, it is necessary to subject the lower layer surface to activating treatment, such as alkali treatment, plasma treatment or corona discharge treatment, for increasing the density of active groups on the surface before the formation of the upper layer.
An attempt is also made for using a lower layer made of an organic material such as polyethylene, polycarbonate or polymethyl methacrylate; making the surface of the lower layer hydrophilic by such a method as plasma treatment or corona discharge treatment; and forming an upper layer made of the same alkoxysilane as described above on the surface of the lower layer. In this case, however, the adhesion is far poorer than in the case that the above-mentioned inorganic material is used as the lower layer. Thus, a sufficient durability is not obtained.
In the case that a substrate to be hard-coat-treated is made of resin, according to the above-mentioned method in which an inorganic material such as SiO2 is used as the lower layer, it is very difficult to obtain the abrasion resistance of the hard coat. When the layer made of the inorganic material such as SiO2 is formed on the surface of the resin substrate, the thickness of the film which can be formed is at most about several hundred nanometers. It is difficult from the standpoint of the production process thereof to make the film thickness larger than such a value. Even if such a film can be formed, the inorganic film self-breaks easily since a difference in elastic modulus or thermal expansion coefficient between the inorganic film and the substrate is remarkably large. It is however difficult that the inorganic film having a thickness of several hundred nanometers gives a sufficient abrasion resistance. It is also difficult to obtain a sufficient adhesion between the resin substrate and the inorganic film. Consequently, the inorganic film is easily peeled. From this viewpoint, it is difficult to obtain a sufficient abrasion resistance, as well.
Therefore, in the case that the substrate to be hard-coat-treated is made of resin, it is necessary to form a primer layer having a high elastic modulus on the resin substrate, form a lower layer made of the same inorganic film as described above on the primer layer, thereby keeping the adhesion between the resin substrate and the inorganic film and the strength of the inorganic film, subject the surface of the lower layer to activating treatment, and form an upper layer made of the same fluorine-based alkoxysilane as described above on the lower layer surface. Since it is necessary to form the three layers successively in this way, the productivity is very poor.
Japanese Laid-open Patent Publication No. 9-137117(1997) discloses a method of applying, onto a surface of a resin substrate, a composition comprising a polymerizable compound having in the molecule thereof at least two (meth)acryloyloxy groups and inorganic compound fine particles such as silica fine particles; photo-polymerizing and curing the polymerizable compound by irradiation of active energy rays; subjecting the surface of this cured film to corona treatment or plasma treatment; and then applying, onto the treated surface, a silane compound having in the molecule thereof at least one group which can generate silanol group by hydrolysis, thereby forming a silane compound coat having an improved adhesion to the above-mentioned cured film. In this case, in order to keep the adhesion between the silane compound coat as the upper layer and the cured film as the lower layer, it is likewise necessary to subject the cured film surface to corona treatment or plasma treatment.
In the case that about a protective layer of the above-mentioned magneto-optical recording medium a liquid lubricant such as perfluoropolyether or polydimethylsiloxane is applied onto the surface of an organic protective layer to form a lubricant film, the adhesion between the organic protective layer and the liquid lubricant film may not be considered very much since the lubricant is a viscous liquid. However, there is a possibility in that the lubricant is Decreased by sliding a magnetic field modulating head repeatedly for a long term or the lubricant volatilizes little by little in storage of the recording medium over a long term. In this method, therefore, it is desirable that the lubricant is firmly fixed on the organic protective layer surface.
Meanwhile, in order to obtain anti-staining property, it is necessary to give water repellency or oil repellency to the surface of a protective layer, as described above. However, this manner is not necessarily sufficient. The operation of wiping off adhering stains is generally carried out by users. Therefore, in order that users can feel that the operation of wiping off stains is easy at the time of carrying out this operation, it is necessary to decrease the frictional coefficient of the protective layer surface. Relationship between the anti-staining property of an article and the frictional coefficient thereof has hardly been pointed out so far. In reality, however, in order to give anti-staining property, it is essential to make the frictional coefficient low as well as give water repellency and oil repellency.
By making the frictional coefficient of the surface low, an impact caused when a hard projection contacts the surface can be slipped away; therefore, the generation of scratches can be suppressed. Accordingly, from the standpoint of improving the scratch resistance of the hard coat, it is required to make the frictional coefficient of the surface low, as well.
Japanese Laid-open Patent Publication Nos. 6-211945 (1994) and 2000-301053 disclose the formation of a hard coat layer by: applying, onto a substrate, a composition wherein fluoroalkyl acrylate and an acrylic monomer incompatible with this are dissolved at a given ratio in a solvent capable of dissolving the two; and irradiating electron rays onto the composition immediately after the application so as to cure the composition. According to these publications, by the application of the composition into a thickness of 1 to 15 μm and the irradiation of the electron rays immediately after the application, the solvent is instantaneously vaporized. Additionally, the fluoroalkyl acrylate compound and the acrylic monomer are localized so that the composition is cured in the state that the fluoroalkyl acrylate is distributed unevenly in the surface of the coat.
However, according to the two publications, it is necessary to irradiate the electron rays onto the composition so as to cure the composition instantaneously after the application of the composition and before the uneven distribution based on the volatilization of the solvent because the composition containing the components incompatible with each other is used. Accordingly, the timing of irradiating the electron rays after the application is difficult and the method for the application is restricted very much. Coating methods in which the evaporation rate of the solvent is large, for example, spin coating cannot be used.
A most serious problem in the methods disclosed in the publications is that there is a high possibility in that since the solvent is vaporized at the same time when the electron rays are irradiated, the solvent in the cured coat cannot be completely removed. In the publications, it is not at all examined whether the solvent is completely removed from the cured coat or not. In the case that a very small amount of the solvent remains inside, no problem is caused immediately after the formation of the hard coat but there is a possibility in that the coat is cracked or peeled after the use of the article with the coat over a long term. The hardness also becomes insufficient. Thus, a warp of the substrate on which the hard coat is formed is apt to increase gradually.
In the method of vaporizing the solvent at the same time when the electron rays are irradiated, the cured coat is apt to have a porous structure. Thus, the hardness thereof is insufficient and, further, the optical property may deteriorate. Accordingly, even if no problem is caused in the case of applying this method to the production of familiar articles, it is difficult to apply the method to the production of articles for which a very high optical property is required, for example, an optical lens or an optical recording medium.
In short, a hard coat wherein anti-staining property, lubricity and abrasion resistance are simultaneously realized at high levels has never been known so far.